Collection classes:

A collection is an object that can be used for storing, retrieving and manipulating groups of items. These items may be primitive data or objects. If you have studied other computer languages, you may be familiar with broadly the same idea under a different name, such as container class or data structure class.

Collections typically represent data items that form a natural group, like a mail folder (a collection of letters), the icons in a screen window, or a telephone directory (a collection of name-to-phone-number mappings).

Homogeneous collections store data that is all of the same type - for example, an array of int can store only int values. Heterogeneous collections may contain data of a variety of types.

This requires some more advanced concepts such as abstract classes and interfaces, which we shall explain in later sections.

The aim of this section is to examine two relatively simple collection classes from the Java Collections Framework, ArrayList and HashMap, and introduce some general concepts that apply to most collections. Later in this unit, we shall discuss the Java Collections Framework in more detail.

The ArrayList class is a more fiexible alternative to arrays. Objects of class HashMap can store pairs of linked items in key-value pairs and allow efficient retrieval of the value associated with a given key. The ArrayList and HashMap classes should be used instead of the Vector and Hashtable classes, respectively - the latter were widely used in older Java code.

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